How to implement persistent mongoDb connection in my express JS app?

I am using mongoose as database for my node/express js project. I had no problem until I decided to migrate my app to AWS Lambda. There I need to create a persistent connection to avoid my app to connect to mongodb each time a function is invoked.

So, I've checked the mongoose docs but it's unclear to me how to integrate this with my current code :

app.get("/", (req, res) => { res.send("Hello World!") });
app.post("/addMessage", myApp.addMessage);
app.listen(PORT, () => {});

module.exports.handler = serverless(app);

I've tried this, but it did not work

let conn = null;
module.exports.handler = async (event, context) => {
  context.callbackWaitsForEmptyEventLoop = false;

  if (conn == null) {
    conn = mongoose.createConnection(URI, {

      bufferCommands: false,
      bufferMaxEntries: 0
    });

    await conn;
  }

    serverless(app)
};

Any suggestion ?